Precision machine vision inspection systems (or “vision systems” for short) are used to obtain precise dimensional measurements of objects and to inspect various other object characteristics. Such systems may include a computer, a camera and optical system, and a precision stage that moves to allow workpiece traversal and inspection. One exemplary prior art system, characterized as a general-purpose “off-line” precision vision system, is the QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published September 1996, each of which is hereby incorporated by reference in their entirety. This type of system uses a microscope-type optical system and moves the stage so as to provide inspection images of either small or relatively large workpieces at various magnifications.
General-purpose precision machine vision inspection systems are generally programmable to provide automated video inspection. Such systems typically include GUI features and predefined image analysis “video tools” such that operation and programming can be performed by “non-expert” operators. For example, U.S. Pat. No. 6,542,180, which is incorporated herein by reference in its entirety, teaches a vision system that uses automated video inspection including the use of various video tools.
The machine control instructions including the specific inspection event sequence (i.e., how to acquire each image and how to analyze/inspect each acquired image) are generally stored as a “part program” or “workpiece program” that is specific to the particular workpiece configuration. For example, a part program defines how to acquire each image, such as how to position the camera relative to the workpiece, at what lighting level, at what magnification level, etc. Further, the part program defines how to analyze/inspect an acquired image, for example, by using one or more video tools such as autofocus video tools.
Video tools (or “tools” for short) and other graphical user interface features may be used manually to accomplish manual inspection and/or machine control operations (in “manual mode”). Their set-up parameters and operation can also be recorded during learn mode, in order to create automatic inspection programs, or “part programs.” Video tools may include, for example, edge-/boundary-detection tools, autofocus tools, shape- or pattern-matching tools, dimension-measuring tools, and the like.
In some applications, it is desirable to operate an imaging system of a machine vision inspection system to collect an image with an extended depth of field (EDOF), such that the depth of field is larger than that provided by the optical imaging system at a single focus position. Various methods are known for collecting an image with an extended depth of field. One such method is to collect an image “stack,” consisting of a plurality of congruent or aligned images focused at different distances throughout a focus range. A mosaic image of the field of view is constructed from the image stack, wherein each portion of the field of view is extracted from the particular image that shows that portion with the best focus. However, this method is relatively slow. As another example, Nagahara et al. (“Flexible Depth of Field Photography.” Proceedings of the European Conference on Computer Vision, October 2008.) discloses a method wherein a single image is exposed along a plurality of focus distances during its exposure time. This image is relatively blurry, but contains image information acquired over the plurality of focus distances. It is deconvolved using a known or predetermined blur kernel to obtain a relatively clear image with an extended depth of field. In the method described in Nagahara, the focal distance is altered by translating the image detector along an optical axis of an imaging system. As a result, different focal planes are focused on the detector at different times during exposure. However, such a method is relatively slow and mechanically complex. Furthermore, altering the detector position may have detrimental effects on repeatability and/or accuracy when it is used for acquiring fixed focus inspection images, which must be used for precision measurements (e.g., for accuracies on the order of a few micrometers) and the like. An improved method for providing an extended depth of field (EDOF) image is desirable, which may be performed at high speed without relying on mechanical translation of optical components.